The invention relates to an evaluation circuit and, in particular, to a device for determining the point of passage of a moving object through a measuring plane.
Such an evaluation circuit can be used, for example, in a device for determining the position of a point on a ballistic curve within a given measuring plane as disclosed in German Offenlegungsschrift No. 2,402,204. In this device, for example according to FIGS. 5 and 6, two cameras are arranged at a known distance from one another and are inclined with respect to one another. The cameras are each provided with a row of photodetectors in their image planes. The signals from the individual detectors are processed in an evaluation circuit (not described in detail in the patent) and are then fed to a computer for determination of the position of the passage of a projectile through the measuring plane.
The detection, for example, of shading of one or a plurality of adjacent detectors in a row as a result of passage of the projectile--it is here assumed that the measuring plane is illuminated--is difficult when the illumination changes.
It is the object of the invention to provide an evaluation circuit which makes the compilation of such detector signals more reliable even under changing external environmental conditions.
In accordance with the invention, apparatus is provided for evaluating signals generated by first and second spatially separated rows of photoconductors positioned with respect to a measuring plane. The outputs of the photoconductors in each of the rows is scanned by a sampling circuit at a predetermined rate, and the outputs of each sampling circuit coupled to at least one evaluation circuit. Each of the evaluation circuits includes an analog-to-digital converter coupled to the output of the associated sampling circuit, and a pair of digital delay circuits having their inputs alternately coupled to the output of the A/D converter. A comparator is provided for comparing a threshold signal generated by a reference generator with the outputs of the digital delay circuits, the comparator generating a digital signal having a value corresponding to the difference between the ratio of the signals at the outputs of the delay circuits and the threshold signal. A computer coupled to the outputs of the comparators in each of the evaluation circuits determines the coordinates of an object traversing the measuring plane.
In accordance with the invention, which evaluates the difference between successively scanned signals from the individual detectors, the reliability with which a momentary event to be evaluated is detected is much greater. A delay of n.multidot.T can be effected in the analog region by means of an analog shift register, for example, a so-called bucket chain circuit. Or, the signals can be converted to digital signals which are then stored temporarily. Due to the same signal influence in both channels, it is of advantage for the two signals being compared to also convert the directly used signal and to then reconvert it (D/A conversion).
A further improvement in signal evaluation is realized, according to a further feature of the invention, in that the series of analog signals obtained from the detectors are fed to a peak value sensor and the output signal of this peak value or average value sensor is used as a comparison value to which the signals from the detectors are related (standardization). This relating can be effected directly in the analog/digital converter.
Since the rows of detectors, e.g. so-called diode arrays, preferably include a large number of individual detectors, a further feature of the invention provides that a plurality of, e.g. four, partial evaluation circuits are provided in parallel and their inputs are connected to the individual detectors of detector groups. If four partial evaluation circuits are used, one partial evaluation circuit is connected in succession, for example, with the first, fifth, ninth etc. detector of the row, another partial evaluation circuit is connected in succession with the second, sixth, tenth etc. detector of the row, etc. In that way, it is possible to either lower the sampling frequency or increase the frequency of sampling. The sampling frequency may lie at several MHz. However, it is also possible to associate, for example, the first 256 detectors with the first partial evaluation circuit, the next 256 detectors with the second partial evaluation circuit, etc.
If the number of partial evaluation circuits employed is a, their a simultaneously appearing signals must be converted into consecutively arriving signals. For this purpose, short-term memories having different lengths of storage times are provided in (a-1) partial evaluation circuits so as to be able to line up the signals one after the other within the available clock pulse time. The result is a sequence of signals whose appearance depends on the partial signals (0 or 1).
For the further evaluation of the n signals thus obtained, these signals are fed to a memory having n memory locations. There also is provided a counter which counts from 1 to the highest number of the n memory circuits or of the number of detectors in a row. If a change in light occurs in the measuring plane, e.g. shading by, for example, a projectile, several juxtaposed detectors experience a change in the impinging light intensity. Thus their signals are changed and the evaluation circuit generates different signals (e.g., a 1 each time)--as described above--at the locations associated with these detectors compared to the locations associated with the other detectors. The above mentioned memory circuit in conjunction with the counter now records the counting position of the counter at which this signal change occurs and the position at which it disappears again. Thus, it is determined, for example, that the signal change occurs at counter position b (of n possible counter positions) and that it disappears again at e.g. (b+10). It has now been determined that detectors b through b+10 of the row have noted shading, for example.
If--as mentioned above--a measuring plane is monitored by two cameras each having such a row of detectors and if the above-described evaluation circuits are connected thereto, the results obtained for the two rows of detectors are such that the coordinates of the passage point can be calculated according to known relationships.